1. Field of the Invention
This invention relates to the protection of temperature-sensitive material with insulation and heat sink compounds.
2 Description of the Prior Art, and Other information
Many items of commerce are subject to degradation or destruction by excessively high temperatures, e.g., single-use clinical thermometers, irreversible temperature indicators, food, and enzymes, antigens, antibodies, or protein substances used in immunoassays or agglutination tests, and other biological or organic substances such as vaccines, sera, etc. Exposure of clinical thermometers for example, to temperatures above 96.degree. F. will cause them to "fire", i.e., to record the exposed temperature and become unusable for further temperature measurement. Products such as clinical thermometers are presently shielded from the adverse effects of high shipping and storage temperatures through heavily insulated shipping cartons containing a "salt foam" formed with sodium sulfate decahydrate as a heat sink material with a relatively high latent heat of fusion.
U.S. Pat. No. 4,425,998 discloses a shell within a shell construction, wherein a 1" to 2" foam insulating shell surrounds and protects an inner shell of "salt foam" having a high latent heat of fusion. The "salt foam" was prepared by melting a compound of sodium sulfate, absorbing the solution into an open cell foam such as phenol-formaldehyde, containing the solution and foam within a polyethylene bag and cooling the solution to form sodium sulfate decahydrate, also known as Glauber's salt. By providing enough insulation to provide a rate of heat transfer through the insulation that is lower than the rate of heat absorption by the compound, a synergistic effect was obtained which significantly extended the period of time in which the thermolabile goods were protected within the carton. While this construction was effective for a single cycle, upon melting and resolidifying, part of the sodium sulfate precipitated out of solution as a particulate sediment, leaving a dilute solution above, which never completely resolidified. In addition, because of the concentrations involved, the solution would partially resolidify as a mixture of sodium sulfate solution and sodium sulfate decahydrate, with multiple incongruent melting points, which adversely effected the performance of the salt foam.
U.S. Pat. No. 4,237,023 to Johnson, et al. discloses an Aqueous Heat Storage Composition Containing Fumed Silicon Dioxide and Having Prolonged Heat-Storage Efficiencies. One of the phase change salts disclosed by this patent is sodium sulfate decahydrate.
U.S. Pat. Nos. 4,187,189; 3,986,969; 2,989,856 and 2,677,664 all issued to Maria Telkes disclose a variety of sodium sulfate decahydrate compositions, one of which, disclosed in 3,986,969 uses a nucleating agent such as borax and a thixotropic agent such as hydrous magnesium aluminum silicate (attapulgus clay) to form a gel like suspension having a heat of fusion of more than 50 BTUs per pound (28 cal/gm).
"Solubilities of Inorganic and Metal Organic Compounds" by A. Seidell and W. F. Linke, American Chemical Society, Washington, D.C., 1965 complies a number of phase diagrams of salt hydrates and other systems from which high latent heat of fusion materials may be selected. Some of these are also set forth in "Thermochemistry of Salt Hydrates", N.T.I.S. Report P.B 227966 (1973) on pages 71-79.